Immunochromatographic strip formats have become increasingly popular for qualitative and semiquantitative assays which use visual detection schemes. This type of assay involves the application of a liquid test sample suspected of containing the analyte to be detected to an application zone of an immunochromatographic test strip. The strip is comprised of a matrix material through which the test fluid and analyte suspended or dissolved therein can flow by capillarity from the application zone to a capture zone where a detectable signal, or the absence of such, reveals the presence of the analyte. Typically, the strip will include means for immunospecifically binding the analyte to be detected with its specific binding partner which bears the detectable label. The label may be one that is visible to the naked eye such as colloidal metal particles or colored latex, an enzyme that forms a visible signal when contacted with a suitable substrate or one that is detectable only with the use of an instrument such as a chemilumenescent or radio active label. In one such scheme, the strip contains an enzyme labeled, mobile binding partner for the analyte which is in a sample application zone. If analyte is present in the test sample, it will combine with its labeled binding partner to form a complex which will flow along the strip to a detection zone which contains a substrate for the enzyme label which is capable of providing a colored response in the presence of the enzyme. The strip may contain a zone in which analyte is immobilized, so that labeled binding partner which does not combine with analyte, due to the absence of analyte in the sample, will be captured and thereby inhibited from reaching the detection zone. There have been published various modifications of this technique, all of which involve some competitive specific binding system in which the presence or absence of analyte in the test sample is determined by the detection or lack thereof of labeled binding partner in the capture zone.
An alternative to the above described immunometric assay which detects the free labeled antibody is the so called sandwich format in which the capture zone contains immobilized antibodies against an epitope of the analyte to which the labeled antibody is specific. In this format, there is formed a sandwich of the analyte between the immobilized and labeled antibodies to provide the detectable signal in the capture zone.
Not all of the schemes for immunochromatography rely on an enzyme labeled binding partner/enzyme substrate for providing the signal for detection of the analyte. In U.S. Pat. No. 4,806,311 there is disclosed a multizone test device for the specific binding assay determination of an analyte and an immobilized binding partner for the analyte together with a capture zone for receiving labeled reagent which migrates thereto from the reagent zone. The capture zone contains an immobilized form of a binding substance for the labeled reagent. The labeled reagent bears a chemical group having a detectable physical property which is detectable on the basis of such physical property, so that it does not require a chemical reaction with another substance in order to be detected. U.S. Pat. No. 4,703,017 describes the use of visible particulate labels for the receptor. Various particulate labels such as gold sol particles and visible dye containing liposomes are mentioned.
European Patent 0 291 194 discloses an immunochromatographic strip of the type presently under consideration. In describing the receiving member this patent states that it can be made from any bibulous, porous or fibrous material including porous plastics such as polypropylene, polyethylene, polyvinylidine fluoride, ethylene vinyl acetate, acrylonitrile or polytetrafluoro-ethylene. While the patentees recognize that some of these materials are hydrophobic, they suggest pretreating the material with a surfactant to reduce the inherent hydrophobicity. The prior art strips typically employ a porous receiving member to absorb the test fluid rapidly so that the strip will quickly become fully wetted when dipped into a test fluid such as urine. However, this wettability of the test strip can become problematical when sample volume is critical such as when the fluid test sample is whole blood obtained from a finger prick. In the case of test strips having a hydrophilic sample receiving member, there is generally a need for higher sample volumes since the hydrophilic member will take up some liquid, depending on its porosity, before sample can reach the next section of the strip. One way to minimize sample volume is by reducing the dimensions of or by miniaturizing the test strip. This approach may not always be suitable due to certain constraints on the dimensions of an encasing cassette for the strip or on an instrument such as a reflectance spectrometer used to read the strip which may not be sufficiently sensitive in some cases.
It would be desirable and it is an object of the present invention to provide an immunochromatography strip of the type described above in which the fluid test sample is drawn from the strip's receiving member to its reagent portion with reduced sample volume.